Recently, there has been an increased demand of developing the UWB technique to realize a high speed data transmission at several hundreds of mega bits per second within a permitted frequency range of 3.1 GHz to 10.6 GHz. In order to comply with regulations with regard to the UWB communication, a bandpass range is restricted to be 500 MHz or 25% of a central frequency, and is different from a specific bandpass around 1.6 GHz given to the GPS (global positioning system). Therefore, the UWB application necessitates a bandpass filter with good near-band rejection.
Japanese Patent Publication (JP 2005-295316 A) proposes a bandpass filter available for the UWB application. The proposed bandpass filter includes a multiplicity of ring resonator. The ring resonators are set to have different attenuation frequency ranges, and are coupled to obtain a desired wide bandpass. However, since each ring resonator is inherently given a relatively narrow rejection band, the combination of the ring resonators is likely to pass undesired frequency or fail to attenuate the undesired frequency to a practically admissible level. Further, since each of the ring resonators is required to have a diameter of up to 1 cm for use in the UWB application, the combination of the ring resonator occupies a relatively large space and is therefore not suitable for incorporation into a miniaturized device.